It is well known that the lubricating capabilities of transmission oils greatly impact the reliability and durability of the transmission. The lubricating ability degrades or reduces with transmission operation and over time. Accordingly, most transmission manufacturers provide guidelines for determining when the transmission oil should be changed. Such guidelines typically are stated alternatively with respect to a measurable period of operation or a period of time.
Typically, the frequency of necessary oil changes is determined by the severity of the service to which the transmission is put, considered in light of the filter equipment installed. Though most transmissions have associated guidelines for the frequency of oil changes or service, more frequent changes are generally required when the transmission operation is subject to high levels of contamination or overheating. Indeed, depending upon the transmission operating conditions, the oil change interval may be as short as 50 percent of the guidelines or may exceed the guidelines by 200 percent or more. It has been found that excessive degradation of the transmission oil occurs at high temperatures. At elevated transmission oil temperatures, antioxidants in the oil become depleted and the oil becomes more viscose and acidic due to the oxidation.
In the prior art, it has been the general rule that transmission oil changes have taken place at periodic intervals measured either on the basis of time as measured by the calendar, or on the basis of operation, as measured either by the number of miles the vehicle has traveled or the number of hours it has operated. In such systems, the oil change has generally taken place based on either time or operation, whichever criteria is satisfied first. However, employing the prior art techniques, no consideration was given to one of the most critical factors, the operating temperature of the transmission oil.
Since the prior art relied upon either a period of operation or a chronological measurement for determining the need for oil change, the technique was typically inefficient and ineffective. Oil changes would rarely be undertaken at the optimum time, but would be either made too early or too late, both being costly to the operator. Additionally, the prior art simply did not take into account the operating temperature of the oil and the impact that such temperature would have on oil degradation. Finally, the prior art did not provide any means for automatically indicating to the vehicle operator the need for the change, nor did it provide the operator with a means for periodically monitoring the state of the transmission oil system and any approaching need for change or service.